1. Field of the Invention
The present invention relates to an automatic frequency control (AFC) system, an operation control method thereof and a mobile communication device using the same. More particularly, the invention relates to the AFC system for matching a reference frequency of a base station and a reference frequency of the mobile communication device in a mobile communication system.
2. Description of the Related Art
An example of the conventional AFC system of this kind is shown in FIG. 5. Referring to FIG. 5, TCXO (Temperature Compensated Crystal Oscillator) 1 is a voltage controlled oscillator generating a reference frequency of a mobile communication device. The reference frequency generated by the TCXO 1 is determined by a TCXO control signal fed from an AFC portion 2. A radio portion 3 uses a reference signal of the reference frequency output from the TCXO 1 to generate a local oscillation frequency by a not shown PLL (Phase Locked Loop) circuit to convert a carrier wave transmitted from the base station into a base band signal.
A digital signal processing portion 4 performs digital processing by using the base band signal from the radio portion 3. By demodulating the signal in a base band signal processing portion (demodulator) 41, communication is enabled. On the other hand, the digital signal processing portion 4 generates a frequency error information between the base station and the mobile communication device in a frequency error measuring portion 42 to feed to the AFC portion 2.
The AFC portion converts the frequency error information obtained by the digital signal processing portion 4 into a TCXO error signal in a TCXO error converting portion 21 and generates a TCXO control signal depending upon the TCXO error signal in a TCXO control portion 22 in order to match the reference frequency of the mobile communication device with the reference frequency of the base station. Since the reference frequency of the mobile communication device has to constantly match with the reference frequency of the base station, AFC control by the AFC portion has to be constantly active whenever the carrier wave is present.
FIG. 6 is a flowchart showing operation of the AFC portion 2, in which the reference frequencies of the mobile communication device and the base station are monitored (step S61). When a frequency error is smaller than a predetermined threshold value, it is regarded as AFC locked condition to stop updating of the TCXO control signal (steps S62 and S63). On the other hand, even in AFC locked condition, monitoring of frequency error is continued (step S61). When frequency error becomes greater than or equal to the threshold value, it is regarded as AFC unlocked condition to resume control for matching the reference frequency of the mobile communication device with the reference frequency of the base station (steps S62 and S64).
Here, an example of a frequency error measuring method in the frequency error measuring portion 42 of the AFC portion will be discussed. From the base station, a known symbol pattern is fed. Using this known symbol pattern, the frequency error of the mobile communication device is calculated. Referring to FIG. 7, there is shown an example of the known symbol pattern to be fed from the base station. When (0, 0) as a first symbol pattern is transmiited from the base station, and when the error is not present between the reference frequencies of the mobile communication device and the base station, the first symbol and a second symbol from the base station are demodulated as the same vector on an I-Q coordinates as shown in FIG. 8.
However, when the reference frequency of the mobile communication device is shifted to cause error, the first symbol and the second symbol become different vectors on the I-Q coordinates and an offset angle θ between the vectors corresponds to the frequency error as shown in FIG. 9. By converting the frequency error θ into a control voltage of the TCXO 1 and controlling the TCXO 1, θ=0 is established to match the reference frequencies of the mobile communication device and the base station. It should be noted that since the frequency error measuring system discussed with reference to FIGS. 7 to 9 is known in the mobile communication system, further detailed discussion will be eliminated.
In the conventional AFC portion 2 shown in FIG. 5, the reference frequency of the TCXO 1 is constantly controlled so that AFC operation is inherently performed even when communication of the carrier wave between the base station and the mobile communication device is interrupted due to influence of building, tunnel or the like and thus communication is stopped. Then, erroneous recognition of frequency error information is caused due to presence of noise component or the like to cause AFC unlocked condition and TCXO is controlled by the frequency error information again generated due to presence of noise component or the like to possibly cause significant shifting of the reference frequency of the mobile communication device.
When the reference frequency is significantly shifted, pull-in speed of AFC can become low or, communication with the base station can become impossible.
Here, reference is made to Japanese Unexamined Patent Publication No. Showa 61 (1986)-73416, in which is disclosed a technology for interrupting updating of AFC data when communication in the communication device is interrupted. When communication is interrupted, it is highly possible that AFC is already offset to possibly hold the offset. Interruption of communication is caused for bad condition of received wave. When AFC is performed with bad received wave, frequency offset can be caused.